The present invention relates to a piezo-electric/electrostrictive film type element, and more specifically, it relates to a piezo-electric/electrostrictive film type element which can be suitably used as a sensor, a filter or the like.
In recent years, as one of mechanisms for detecting a pressure change or the like of a void formed in a substrate, a piezo-electric/electrostrictive film type element is known in which a piezo-electric/electrostrictive working portion is formed on the outer surface of the void, and the piezo-electric/electrostrictive working portion functions as a mechanical/electrical converter to detect the volume change of the void by the pressure change in the void. Such a piezo-electric/electrostrictive film type element has been utilized as, for example, a sensor or a filter. The pressure change of an atmosphere to be detected which has been introduced through an opening expands or contracts the volume of the void, and at this time, the strain of the wall of the void gives a mechanical change to the piezo-electric/electrostrictive working portion and this mechanical change is then converted into an electrical signal by the piezo-electric/electrostrictive working portion.
FIG. 5 shows one embodiment of the conventional and known piezo-electric/electrostrictive film type element used as the sensor. In this drawing, the pressure change of an atmosphere to be detected which has been introduced through an opening 26 expands or contracts the volume of a void 30, and at this time, the strain of the wall of the void gives a mechanical change to a piezo-electric/electrostrictive working portion 22 and this mechanical change is then converted into an electrical signal by the piezo-electric/electrostrictive working portion 22.
A piezo-electric/electrostrictive film type element 20 is constituted of a ceramic substrate 21 and the piezo-electric/electrostrictive working portion 22 integrally attached to ceramic substrate 21. Furthermore, this ceramic substrate 21 is integrally constituted of a thin and planar closure plate 23, a base plate 24, and a spacer plate 25 sandwiched between these members 23, 24.
The spacer plate 25 is provided with a window 28, and the spacer plate 25 and the base plate 24 are superposed on each other so that this window 28 may be communicated with the opening 26 formed through the base plate 24. Moreover, on the side of the spacer plate 25 opposite to the side thereof superposed on the base plate 24, the closure plate 23 is superposed, and the opening of the window 28 is closed by the closure plate 23.
Thus, a void 30 is formed in the ceramic substrate 21.
On the outer surface of the closure plate 23 of the ceramic substrate 21, the piezo-electric/electrostrictive working portion 22 is arranged on a position corresponding to the void 30. Here, the piezo-electric/electrostrictive working portion 22 is constituted of a lower electrode 31, a piezo-electric/electrostrictive layer 32 and an upper electrode 33.
The conventional and known piezo-electric/electrostrictive film type element is constituted as described above, and a plan view of the piezo-electric/electrostrictive layer 32 and the void 30 is shown in FIG. 6. For the enhancement of a mechanical/electrical conversion efficiency, the piezo-electric/electrostrictive layer 32 is formed somewhat larger than the void 30 so that the piezo-electric/electrostrictive layer 32 may cover the whole surface of the void 30, a manufacturing precision being taken into consideration.
However, if the piezo-electric/electrostrictive working portion 22 is formed so as to cover the whole plate area of the void 30, it has been considered that a vibration mode based on the fluctuation of the piezo-electric/electrostrictive working portion 22 is not uniform in the central portion and the end portions of the void 30 owing to the restriction of these corner portions. In consequence, a certain gap has occurred between the characteristics of a simulation at the design of a sensor, a filter, an acoustic oscillator or the like and those of the actual product.
Furthermore, in the case that the piezo-electric/electrostrictive working portion 22 is arranged so as to cover the whole planar area of the void 30, the electrode films (the upper and lower electrodes) 31, 33 and the piezo-electric/electrostrictive layer 32 can be formed on the outer surface of the closure plate 23 by a known film formation method, for example, a means such as a screen printing method or a spray method, followed by a heat treatment. At this time of the heat treatment, the respective films contract due to the firing, but both the end portions of the void 30 contract much less (i.e., they are not densified) as compared with the central portion thereof owing to the restriction at both the ends portions of the void 30. As a result, some problems are present. For example, a firing degree is not uniform on the whole, and a large stress remains in both the end portions. Such an ununiformed firing degree of the void 30 has a bad influence on the vibration mode of the piezo-electric/electrostrictive working portion 22.